Electrophoretic gels, usually comprising hydrogels such as agarose or polyacrylamide, are used for the separation of nucleic acids, proteins and other macromolecular compounds. The sample to be separated is placed at one end of the gel and a direct electric field is applied between the ends of the gel causing the components of the sample to migrate through the gel at rates dependent upon their molecular size and charge.
A mixture of components to be separated is normally introduced into one of a number of small wells formed in an upper edge of the gel before the electric current is applied. It is usual to run a number of such mixtures simultaneously on an electrophoretic gel in a side by side arrangement. For this purpose one mixture is placed in each of a series of wells formed in the upper edge of the gel.
It is conventional to form electrophoretic gels by juxtaposing a pair of glass plates in a slightly spaced apart side-by-side relationship and filling the space there between with a liquid which can set and form an electrophoretic gel. The side edges of the space between the glass plates are, in this case, sealed with adhesive tape or a similar material, when the gel is poured a comb is placed in the upper end of the space between the glass plates. After the gel has set the comb can be withdrawn leaving in the top of the gel a series of spaced apart wells, each well having been defined by one tooth of the comb. A tongue of gel remains between the glass plates separating a pair of adjacent wells.
In more recent years it has been proposed to pre-form electrophoretic gels in cassettes formed of synthetic plastics materials. The side walls of the cassette are formed with integral means to connect them together along the sides of the cassette. In U.S. Pat. No. 5,288,465 ribs are provided on the cassette walls to define wells at one end of the cassette. This arrangement has the disadvantage that the walls of the wells interfere with the smooth flow of electric current through the electrophoretic gel.
In the case of pre-formed gels in a cassette, the use of a conventional comb has the disadvantage that upon withdrawal of the comb the tongues of gel may, with time, show an increased tendency to break away from the remainder of the gel. This results in poorly defined wells. Alternatively, if the tongues of gel are left intact upon withdrawal of the comb they may not be firmly adhered to the side wall of the cassette. This means that the tongues may fold over sideways occluding an adjacent well. In either case the smooth and convenient operation of the cassette is inhibited.
The present invention is directed to an alternative arrangement in which the tongues of gel may be maintained in position in the cassette thereby ensuring clear definition of the wells at all times.